Furnace control



March 18, 1941. A. E. KROGH Funmcn connor. Filed oct. '7; 19;?

Anker E. Kragh INVENTOR 2V f N N, L e.4 a. JLKUL imn L illu. .3v/m. .HK E 1J 1111 f F. D

H C :A 12 A Patented Mar. 1,8, 1.941

UNITED STN-ras PATENT OFI-*Ica d .Y FUBNsirnoL d I l I Anker E. Krogh, Philadelphia, Pa., assigner to The Brown Instrument Gompany, Philadelphia, Pa., l a corporation of Pennsylvania Application October '7, 1937, SerialNo. 1.67,'118

9 Claims.

The general object ofthe present invention is to provide an improved system for controlling temperature conditions in annealing furnaces of the intermittent type, and in other heat treating furnaces in which it is desirable to make the heatand is characterized in particular, by the comparative simplicity of the control apparatus required for regulation of the furnace heating eilect in response to separately measured work and furnace temperatures.

In accordance'with the prese-nt invention a single measuring and control instrument is employed to separately measure work and furnace temperatures and to effect control actions in ac cordance with the measurements.

In the preferred practical form of the present 30 invention, a single controlling potentiometer instrument in association witha periodically operating switch mechanism. measures and records the temperatures to which one furnace and two work temperature thermocouples. are subjected, successively at regular intervals, and, suitably adjusts the instrument control contact of a socalled proportioning control system. The latter includes a reversible-relay motor which is automatically operated to make the rate of heat supply to the furnace under normal operating conditions, that required to maintain an approximately constant predetermined controlling work temperature, except that the furnace temperature, when it exceeds a somewhat higher predetermined temperature, causes the .motor to operate in the direction to reduce the rateof heat supply to the furnace.

The various features of novelty which characterize the presentinvention are pointed out with particularity in the claims annexed to, and forming a part of this specincation. For a better understanding of the invention, however, its advantages and specic objects attained with its use, reference should be had to the accompanying drawing and descriptive matter in which I (Cl. 236-15) A have diagrammatically illustrated and described y a preferred embodiment of the invention.

The one ligure of the drawing is a diagrammatic representation of v'a bell type furnace, and

means for controlling the heat supply to the '5 furnace in response to -work'and furnace temperatures. y

In the embodiment of the invention diagrammatically illustrated by way of example, A reprecents the bell or dome of a furnace of well- 10 known type, comprising a hearth or furnace base B upon which the bell A is seated during the heating operation, and from which it'is removed for the removal ofthe treated work and the introduction of work to be treated. The bell A 15 as ordinarily constructed1comprisesa metal shell and a refractory lining A'. Ordinarily, and as shown, the roof of the bell A carries a bail or cycli2 for engagement by the hook ofva crane i (not shown) employed to lift the bell off, and 2o replace it on the base B. 'I'he work C in the furnace to be annealed or otherwise heat treated, as conventionally illustrated, consists of a pile of metal plates stacked up on the base'B. As shown i the work C is enclosed by an inner bell, hood, or work cover of metal, smallery than and spaced away from the side and roof walls .of the bell A. The hood D rests on the base B. Sealing means D' are provided to prevent ga's leakage between the space enclosed by,V the inner hood or cover D,

and gthe space between the bell A and hood D. The furnace conventionally illustrated inthe drawing,is of the well-known, so called radiant tube" type, inwhich the furnace is heated by radiation from, a plurality of metal tubes E,

each comprising a vertical portion at the inner side and adjacent the vertical wall of the hood A,- and having upper and lower end portions extending horizontally through said wall. Each of the tubes E serves as avcombustiony chamber receiving a z nixture of fuel gas and air for combustion at its lower end, from a supply manifold F at a rate regulated, as diagrammatically illustrained, by the adjustment of a supply valve G. Each tube discharges products of combustion at its upper end. As shown sealing means A3 are provided for sealing the joint between the lower edge of the'hood A and the base B, and other sealing means A4 are provided to prevent leakage i through the openings in the vertical wall of the bell A through which the upper horizontal portions of the tubes E extend, those openings being llarger in cross section than the tubes, to accom-v modate the relative expansion ofthe tubes E and bell A.

In the drawing H and I are'thermocouples responsive to the temperatures of the work adjacent its top and bottom, respectively, and J is a thermocouple responsive to the furnace or tube temperature. As shown the thermocouple J has its hot junction closely adjacent the outer side of the vertical portion of one of the heating tubes E.

A switch mechanism KL is provided to connect the terminals of the three thermocouples in regular succession to conductors I and 2 running to the measuring terminals of a temperature measuring and control'instrument M. As diagrammatically shown, the switch mechanism KL comprises a circular series of similararc shaped contacts H', I' and J', a second series of similarv arc shaped contactsI-I, I2 and J2 and switch arms K and L carried by a constantly rotating shaft N. The thermocouple H has one terminal 1 connected to the contact H', and its second terminal connected to the contact H2, and the two contact arms K and L are so disposed relative to the two sets of contacts, that the arms K and L are simultaneously in'engagement with the contacts H' and H2, respectively, for a suitable fraction, ordinarily little less than one-third, of the time required for each revolution of the shaft N. Similarly the terminals of the thermocouple I are connected to the contacts I and I2, and the terminals of the thermocouple J are connected to the contacts J' and J2, and the switch arms K and L simultaneously engage the contacts I' and I2, respectively, -during a second fraction of each revolution of the shaft N, and engage the contacts J and J3, respectively, during aA third fraction of, each revolution of the shaft N.

The `shaft N is advantageously rotated by a timing motor N', `and may be an extension ofthe shaft N2, diagrammaticaliy'illustrated as the armature shaft ofthe motor N. As shown the motor N' has one terminal of its energizing winding Na directly connected to asupply conductor 3, and the second terminal connected to the cooperating supply conductor 4, through a conductor' 5 and a normally closed switch 6.'

The instrument M shown isa self-balancingpotentiometer instrument, and may well be of the commercial recording potentiometer type,

known as the Brown potentiometer, the general character of which is disclosed in the Harrison Patent No. 1,946,280, granted February 6, 1934. As shown the conductor I is connected to one terminal M2 of the potentiometer slide wire re-` sis'tor M'. 'I'he conductor 2- is connected to one v-termin-a1 of the instrument galvanometer M3.

The second. terminal of the latter is connected to a contact M* in engagement with. and adjustable along the length of, the slide wire resistor M'. As diagrammatically illustrated, the contact M4 is carried by the instrument pen carriage Ms and the latter is adjusted along the length of the resistor M' by the rotation of a helically grooved shaft M in threaded engagement with the carriage M5. 'Ihe shaft MU is rotated as required to rebalance the potentiometer, by a power actuated -rebalancing, relay mechanism Ml.. operating in .the bridge circuit by adjusting magnitude, but opposite in direction, to the voltage of the particular thermocouple then having its terminals connected to the conductors I and 2 through the switch mechanism KL.

If at the beginning of each period during which a particular thermocouple is connected to the instrument M, the voltage of that thermocouple is greater or less than the potential drop between the terr'ninalMn and the contact M4, the. galva- -nometer- M3 will ca use the relay mechanism M"l to rotate the shaft Ms as requiredto adjust the carriage M5 into the position in which the potential drop between .IVF and M4 is equal to said voltage.

Aas

a proportioning system, and includes a so-y called proportioning relay, shown herein as a reversible motor. R. The latter is actuated by voltage variations, or unbalancedue to adjustment of the contact P, in the bridge circuit, to adjust the fuel valve G, and to simultaneously rebalance a contact Q.

As shown, said bridge circuit comprises two slide wire resistors p and q, connected in parallel between branches 1 and l of conductors 3 and 5, respectively. The contact P engages the resistor yp, and is adjusted along the length of thel latter by the link O, on, and proportionally to, each adjustment movement of the carriage M5. The contact Q engages, and is adjusted along the length of resistance q, when the motor R is in operation, through an operating-connection R between the armature of the motor R and the contact Q. Similar coaxial solenoid coils, SA and SB, 'each in inductive relation with an armature S', are connected in series with one another, and in paralllei withthe resistors p and q, between the conductors l and 8. The common Junction or terminal S of the coils SA and SB, is connected by a conductor 9 to the contact P, and by a conductor III to the contact Q.

In the balanced condition of the bridge, the potential drop between the conductor v8 and the contact P is equal to the potential drop between the contact Q and conductor 1, and the current ow is the same in the coil SA as in the coil SB. Any adjustment of the contact .unbalances the bridge, if the latter is previously balanced. The unbalancing of the bridge makes the amount of current ow through one of the coils SA and SB, greater than through the other, and thereby causes movement of the solenoid armature S', in one direction or the other, away from its normal neutral position shown. The armature S' is mechanically connected to a movable switch arm T, which is shifted from the intermediate.

open circuit position shown in the drawing, by

movement of the armature S' out of its neutral position, Into a position in which it engages one or the other of the switch contacts '1" and T2, depending on the direction of the movement of the armature S'. Such engagement of T with T' or T2, is required to permit, but as hereinafter explained, does not necessarily result in, enerf gization of the motor R.

"ItA

An adjustment of the contact P to the right.

as seen in the drawing, with the bridge previcoil SB larger than the current ow through the coil SA, and the armature S will thenV tilt the switch blade T into engagement with contact T'. 0n the assumption that contact Pimoves to the right in responsev to an increase in the thermocouple voltage measured by the instrument M, any operation of the motor R, which may occur as a result o! engagement of switch blade T with contact T2, will .be in the direction togive a closing adjustment to the fuel valve G, through the operating connection R2 between the armature of the motor R and the valve G, and to give a rebalancing adjustment of the 'contact Q to the left, through the operating -connection R. When the unbalancing adjustment of the contact P is to the left, it results in an i adjustment of the switch arm T into engage-` ment with the contact T', and the resultantoperation, if any, of the motor R, is in the direction to give an opening adjustment to the valve G, and to adjust the contact Q to the right.

Operation of the motor R in the direction giving an opening adjustment of the fuel valve G results from the energizing of the motor winding R3, which must then have one terminal connected to the contact T'. l motor in the opposite direction results from the energization of motor winding R4. which must then have one terminal connected to' the` switch contact T2. TheI winding R3 is disconnected Afrom the contact T' when the valve G is given its maximum permitted opening adjustment, by'

the opening of a limit switch R5; and a limit switch R6 is opened to disconnect winding R* and contact T2, when the valve attains its -Iully` closed position. The switches R5 and l'\.f,lormV part of a limit switch mechanism, which may be of any usual or suitable type, andas shown, is actuated by a connection R'I tothe motor armature. The switch blade T is connected tothe supply conductor). The second terminals .of the windings R3 and R4 are connected together.

and both are connected to the conductor-l, and

thereby to the conductor l, when the switch i.

and one, at least, of two switches U andW-are in their closed positions.

The switch U is moved between open and closed positions by a cam N4 carried by the timing motor shaft N1, and the movement of the switch U is so timed relative to the movements of the switch arms K and L,` that the switch U is closed only during intervals in which the work bottom thermocouple I is Vconnected through conductors I and 2 to the intrument M. 'The switch W is closed by a movement of the instrument carriage M5 into a-hig-h temperature position, in which it engages a lever X and tilts the latter from the full line into the dotted line position shown in the drawing. The high ltemperature position of the carriage M5, tilting the lever X into its dotted line position can occur,funder normal operating conditions,.only when the furnacethermpcouple J is connected through conattained, the minimum temperatures of the Operation of the thermocouple J will be much higher-thanthe maximum temperature of either thermocouple H or I. By way of' illustration, merely, it is noted that in the regular'use of one particular Afurnace employed for o ne particular annealing operation, the` maximum' desirable temperature o! the thermocouple J is slightlyin excess of 1700 F., while the maximum temperature of each of the workthermocouples H and I, is between 1300 and 1400 F.

The measured temperature at'which carriage Ms will tilt the lever X, can be adjusted by varying'the length of the link W' connecting the switch W to the lever X, and by adjusting the position'oi the lever X along the length of the grooved shaft M6. As diagrammatically illustrated, the length of the link W' may be varied by means of a tur'n buckle W2 threaded to sections ofthe link W', which it connects. The position of the lever X may be adjusted, as diagrammatically shown,by rotation of a screw X extending through a stationary nut X2 and swivelled to a movable-support X3, on which the lever X yis pivotally supported.

As will be apparent, the operation of the moltor- R in the direction to givethe valve G an opening adjustment, can occur only when the switch T engages contact T' and the limit'switch .Ri andvswitch U are each closed. Since the closure of the switch W results from theumovement of the. carriage M! to the right into a high temperatureposition,the solenoid armature S is not' actuated to .tilt switch blade T intov contact with T', under the condition in which the y switch W is initsclosed position. Furthermore,

the switch blade T can be in engagement with Athecontact T' only during an interval in which the temperature of the work thermocoupleI is.

lower than that required to balance the bridge `with the contact Q in the position corresponding to the previously existing adjustment of the fuel valve G.

The operation oi the motor R in theidirection to give'a closing adjustment tothe valve G can l occur only 4when thelimit switch R'3 and one, at

least, of the switches U and Ware closed,"and

the switch blade T is in engagement with the `contact T. `Aswill be apparent, the' motor R may be operated to. give a closing adjustment to. the valve G, when the furnacetemperature is high enough to result in the closure of the switchV W \by the action ofthe carriage M5 on the lever X,` and when the temperature of the thermocouple I is higher than the temperature corresponding to-the previous existing adjustment ot the valve Gand contact .Q

From what has been said, it will be apparent that the heeft supply .to'the furnace is controlled by .the temperature oi' the work bottom thermocouple I, except as that control ismodified by intermittentr reductions in the rate of heat supply,- made, onthe attainment of a temperature by the furnace thermocouple J whichhas been predetermined as a desirable maximum temperature, vand lwhich results in the closure of the switch W.v The range-of variations in tempera-A ture of the lower work-.thermocouple I, in which the valve G is subjecttoadjustment inresponse to said variations, depends upon the adjustment of the apparatus. For ordinary conditions, I now,

consider it desirable to adjust `vthe apparatus so that, insofar 'as control due to the .thermooouple I ductors I and 2 to the instrument M, since normally when full working temperatures have been is concerned, Ithe 'valve G will be wide open, when the temperature of the thermocouple I is not high enough to cause movement of contact P to the right' or its mid 'posi-tion eiong the resistor p. Such adjustment of the apparatus is eilectedby so relatively proportioning and arranging the connections through which the motor R adjusts the contact Q,valve G, and limit switch R5, that` as the motor R moves the contact Q to the righ-t 'into its mid position, it simultaneously adjusts the valve G into its wide open position and opens the limit switch R5, so that motor R is then. prevented from further movement in the direction to open the valve G and adjust .the contact Q to the right.

The temperature of the :thermocouple I which corrponds .to the mld position of the contact P, may be made anything desired, within practical limits, by suitable adjustments of the appara-tus, as, for exam-ple, by rotating a turn buckle ing the heating up operation, each closing adi 0' in threaded engagement with the sections of the link O, whichit connects. With the apparatus adjusted as above described, .the fuel valve G will be in position to pass the maximum amount of f-uel throughout -the entire lheating up operation, in which the .temperature oi the .ther mocouple I `is increased from its initial atmospheric temperature to the ltemperature corresponding to the mid position of the contact P,

' unless, as may sometimes happen, during the ilnal portion of the'heating up operation, the temperature of the furnace thermocouple J becomes high enough to effect closure of the switch W. Durjustment of the valve G, eflected while the ther- -mocouple J is connected to the instrument N. will be followed by an opening adjustment of the valve G, when the .thermocouple I- is next connected to the instrument M.

With the substantial diiIerence normally preveiling-between the temperatures of the thermocouple I connected to instrument M, the resid/tant' the length of the resistor p, at which the latter' is engaged by .the contact P, when with thermorebalancing motion of the motor R will completely close the valve G. As those skilled in .the art will understand, the distance between the noi-nt p' and the mid point of the resistor pfdetermines what is sometimes called the "lthrottling range of the valve G, i. e., the range of variation of .the controlling temperature of the thermocouple I, required to effect adjustment of the valve G between its wide open and its fully closed positions. With the temperature of the work thermocouple I, within saidrange, valve G will be given an opening or closing adjustment, whenever the switch U is closed, if adjustment of the contact Q to the right or left is then necessa to rebalance the bridgecircuit.

When the temperature oi the thermocouple I is Within its throttling range, as when lower, the closure of the switch W will result in a closing adjustment of the valve G, andthe next closure of the switch U, may result in an adjustment of the valve G in either direction, or in no adjustment at all', depending on the relative positions of the contacts Q and P, when .the position of the letter correspond to .the temperature of thermocouple' I. It thus follows that whenever the furnace temperature is high enough .to close the switch W, the rate of heat supply is jointly dependent on the temperatures of the thermocouples I and J. With 'a lower temperature of the thermocouple J, .the rate of heat supply depends wholly on the temperature of the thermocouple I.

As will be understood by those skilled in the art, the throttling range of .the valve G may be varied by varying .the extent of throttling movement given the movable member of the valve G, relative to the extent of movement 'given the contact Q, by a given extent of movement of the motor R. Such an adjustment can be effected by .parts adjusted by the potentiometer rebalancing mechanism, the instrument M as diagrammatically illustrated, is precisely similar in Vprinciple tothe Brown potentiometer, though in its ordinary form,v that commercial instrument/ .includes mechanical control parts and adjusting provisi0ns,'dii!erent in form from those diagram- 'matically indicated in Fig. 1. Such formal difierences require no further discussion herein, however, because the.'fBrown'potentiometer is widely used and well-known, and also because for the purpose of the present invention, the instrument M may take the form of any one of various potentiometer and other measuring instruments in commercial use.

In referring herein and in the appended claims to the adjustment of the contacts P and Q, the adjustment of these contacts relative to their respective resistors p and q, is contemplated, and such adjustment may be eiIected by moving the resistors while the contacts engaging them are stationary as is done in-some commercial potentiometer instruments as well as by moving the contacts while holding the resistors stationary, as is done in other commercial potentiometer instruments.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed"without departing from the spirit of my invention as set forth in the appended claims and that in somie cases certain features of my invention may be used to advantagevwithout a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. The combination with a heater, of means including a single meter for alternately measuring the temperatureof the heater and the temperature of work heated therein, a device-adjustable to regulate the heat supply to the heater, and controlling means cooperating with the rst mentioned means to adjust said device to increase or decrease said rate, if not already at its maximum or minimum, respectively, on and in accordance with decreases and-increases in the work tempertacts, a heat supply regulator, means'through which said iirst contact is adjusted by and in ac-v 2,285,219 ature measured when the latter exceeds a predetermined temperature, and to reduce said rate,

`if not already at its minimum, whenever the heater temperature measured exceeds asecond predetermined temperature.

2. The combination with a heater and devices respectively responsive, to a temperature of the cordance with the deflection of said element, a control motor adapted to adjust said second con-` tact on and in accordance with the rotation of i the motor-in either direction, controlling means for said motor actuating'thelatter in response to unbalance in said bridge to effect a rebalancing adjustment'of said second contact and to adjust said regulator, when said work temperature responsive deviceis connected to said instrument and when the other device is connected to said' instrument -and` has attained a hpredetermined high temperature 3. The combinationwith aheater and devices respectively responsive to a temperature of the heater and to a temperature ofthe work heated therein, a mleasuring instrument including an element deecting in accordance` with the value of the quantity measured, means for connecting said devices alternately to said instrument, a bridge circuit including energizing conductors and two resistors connected in parallel between said conductors, a first contact engaging and adjustable along one of said resistors, a second contact engaging and adjustable along the second of said resistors, nieans through which said iirst contact is adjusted byI and in accordancewith the deflection of said element, a control motor connected to and adapted to adjust said second contact on and in accordance with the movement of the motor, controlling means for said motor actuating the latter in response to variations in a relation of the potentials of the twocontacts which may be interrupted by' adjustment of said rst contact and restored by the adjustment of said second contact effected by said motor, and a heat supply regulator adjusted by said motor.

4. The combination with a heater of a device responsive to a temperatureof the heater, adevice responsive to a temperature of the work heated which is normally. lower than the iirst mentioned temperature, a temperature measuring instrumient including an element defiecting in accordance with the temperature measured, means for connecting said devices to said instrument at alternate periods, a regulator ada justable to regulate the rate of heat supply to the heater, and means operative only during periods at which the work temperature is measured, for adjusting said regulator to increase or decrease the rate of heat supply to the furnace in accordance with the deflection of said element through a small range of deflection below or above apredetermined work temperature, and means operative only during periods in which the furnace temperature is being measured, and on the deflection lof said. .element into a positionY corre-- sponding to a predetermined temperature higher vthan the maximum temiperaturegi'said range', to decrease said rate of heat supply?" 5. -The combination with a-heater and devices respectively responsive to a temperaturefof` the heater and to-a temperature of the work heated therein, of a measuring instrument includingan yelement deiiecting in accordance with the value of the quantity measured, means for connecting said` devices alternately to said instrumenta con- Itrol bridge circuit. including a 'measuring contact -adapted to be adjusted by and in accordance with thedeilection of said element and on each adjustment to unbalance said bridge circuit, if previously balanced, and a contact adjustableA to rebalance said circuitwhen unbalanced by the adjustment -of said iirst contact, means ,responsive to unbalance of said bridge circuit,a regulator adjustable to regulate the supply of heat to the furnace, a motor adapted when energized to give a re-balancing adjustmentit'osaid second contact and toproduce a corresponding-adjustment of said regulator, a limit switch opened on adjustment of said -regulatorinto its `maximum vheat supply position, a, timingswitch closed during periods in which thework' temperature is measured and open during periods in which the heater temperature is measured, and aswitch closed by said responsive means on unbalance in said cir-` cuit resulting from a decrease in the temperature measured, means energizing the motorhforoperatin'gjin the directionto reebalanc'e"saidcircuit and give a heat supply increasing adjustment to said i regulator when said three .switches are, closedj '1 6. The combination with a furnace and devices respectively responsive to a temperature ofthe y heater and to a temper tureof vthe-'worky heated therein, of a measuring vnstrument including an element deiiecting, in accordance withthe value of the quantity measured, means for connecting said devices alternately to said instrument, a control bridge circuit including a. measuring contact adapted to be adjusted by and in accordance with the deilection of vsaid element and on each adjustment to unbalance said bridge circuit, if preyiously balanced, and a contact adjustable to rebalance said circuit when unbalanced by the adjustment of said first contact, means responsive to unbalance of said bridge circuit, a regulator adjustable to regulate the supply of heat to the furnace, amotor adapted when energized to give a re-balancing adjustment to said second contact and to produce a corresponding adjustment of said regulator, a limit switch opened on adjustment of said regulator into its maximum heat supply position, a timing switch closed during periods in kvwhich the work temperature is measured'and open during periods in which the heater temperature is measured, and a'switchclosed by said responsive means on unbalance in'said circuit resulting from a decrease inthe temperature measured, means energizing, the motor for operation'in the direction to rebalance said circuit and vgive a heat supply increasing adjustment to said regulator when said three switches are closed, a limit switch opened on adjustment of said regulator into its minimum heat supply position, a switch closed by adjustment of said element into a position corresponding to a maximum desired heater temperature, and a switch closed by said responsive means on unbalance in said circuit resulting from an increase in the temperature measured, and means energizing said motor for voperation in a direction opposite to the previously mentioned direction, when the last three mentioned switches are closed.

7. 'Ihe combination with a heater, a device responsive to a temperature ot the heater, and a device responsive `to a temperature ofy the work heated therein, of a measuring instrument including anelement deilecting in accordance with the value of the quantity measured, means for connecting said devices alternately to said instrument, a bridge circuit, a measuring contact adapted to be adjusted by and in accordance with Athe deflection ofv said element and on each adjustment to unbalance said circuit if the latter is. previously in balance, a second contact adapted to be adjusted to rebalance said circuit when un'- balanced by the adjustment oi' said measuring contact, a motor adapted when energized to adn just said second contact and to produce a corre-` sponding adjustment in'- the rate kof heat supply' to said heater, motor controlling means including means cooperating with the means for con`. necting said devices alternately to said measur-4 ing instrument.to energize said motor for operation effecting a rebalancing adjustment of said second contact when the work temperature reatures oi' heater and of work heated therein in upper limit.

aussage regular alternation and progressively increasing or decreasing the rate of heat supply to the heater by small increments as required to. make said rate an inverse function of the work temperature measured lwhen said temperature is within -a throttling range of temperature substantiallysmaller than the full range of work temperature variation, said rate oi' heat supply being a maximum when said work temperatureis below said throttling range except when said heater tempera ature exceeds a predetermined temperature'. and

reducing said rate of heat supply, if not already at its minimum whenever the heater temperature Y measured exceeds said predetermined temperature.

9. In controlling the operation of a batch heater in which the heater Vtemperature and work temperature each increase through a substantial range during a batch heating operation, the methodlwhich consists in measuring the temperatures of theheater and-work heated therein in regularalternation, and in progressively decreasing the rate of heat supply to the heater by small increments when the work temperature measured increases while within a predetermined throttling ,range of temperature. as required Vto make said rate an inverse function ot the work temperature measured, and when the heater temperature measured has attained a, predetermined value higher than the upper limit of said range,

said-rate of heat supply fbeing a maximum when said work temperature is below said throttling range except when said heater temperature exceeds said predetermined value, and in increasing said rate of heat supply only at times at which the work temperature measured isv belowsaid ANKER-E. KROGH. 

